This application is a competitive renewal to study principles of neural organization and function within motor systems. For this purpose, we have focused on the organization of interneurons and motor neurons mediating escape swimming in the mollusc, Tritonia. During the last grant period, we showed that a single network of interneurons can be organized into different circuits, each of which subserves a different motor function, swimming and reflexive withdrawals. Our specific aims for the next grant period are: (1) Although substantial portions of the neural circuitry mediating swimming are known, several important elements are missing. Using a combination of anatomical and electrophysiological approaches, we will identify the missing neurons and characterize their synaptic relationships to the known circuitry. (2) This information will be incorporated into a computer model of the swim system and will be used to evaluate the role of specific neural mechanisms in pattern generation. (3) The pattern of synaptic connectivity within the swim system is extremely complex involving many multi-action synapses. Little is known about what transmitters are involved or how they mediate their multiple actions. A combination of cell isolation and electrophysiology will be used to identify putative transmitters and to characterize their mechanisms of action. A new cell isolation technique will be used that allows direct access to receptors on the neuropilar processes. (4) The ability of the swim system to show behavioral plasticity (habituation and sensitization) will be assessed as the first steps towards a long-term study of cellular mechanisms of learning. Electrophysiological experiments are proposed to identify sites of plasticity and to characterize cellular and synaptic mechanisms.